Acute lung injury (ALI) is characterized by the abrupt onset of respiratory distress and is associated with hypoxemia and diffuse pulmonary infiltrates. The acute respiratory distress syndrome (ARDS) is a more severe form of lung injury and is an important clinical problem in the care of critically ill patients. Using current criteria, the diagnosis of ALI/ARDS requires an assessment of intravascular volume and is thus limited by the difficult and at times imperfect assessment of intravascular volume. This proposal, designed as a prospective cohort study, will evaluate whether bronchoalveolar lavage (BAL) can be used to distinguish patients with permeability pulmonary edema from those with hydrostatic pulmonary edema. In patients mechanically ventilated for respiratory failure, we will obtain a post-intubation sample of edema fluid and follow this with BAL within 24-48 hours. Markers of alveolar inflammation and epithelial injury in the BAL fluid will then be evaluated for their ability to identify patients with permeability edema, using the edema fluid-to-plasma protein ratio as the standard. In addition, other diagnostic modalities, such as pulmonary artery catheter parameters and echocardiographic indices, commonly used in the assessment of intravascular volume will be evaluated for their ability to identify patients with hydrostatic permeability edema. If successful, BAL could be used alone or in concert with these markers of intravascular volume to better identify patients with ALI.